I may have found another possibility. This from an envionmental firm that does studies in the Great Lakes area:
"First of all, have you ever seen a stream, wetland, pond or ditch with a rust-colored substance all over the bottom and sides? Commonly, if you see this substance you may also see an oily film on the surface of the water. Although this looks like pollution, it is actually not. In areas that have iron-rich water, ground water seepage, and low flow, naturally occurring bacteria called Iron Bacteria oxidize iron for energy. The by-product is ferric iron, which becomes iron oxide when it is exposed to air and water. Iron Bacteria are not known to pose any environmental or human health risk."
